Target for use in magnetron sputtering of aluminum for forming metallization films having low defect densities and methods for manufacturing and using such target

ABSTRACT

Improved targets for use in DC --  magnetron sputtering of aluminum or like metals are disclosed for forming metallization films having low defect densities. Methods for manufacturing and using such targets are also disclosed. Conductivity anomalies such as those composed of metal oxide inclusions can induce arcing between the target surface and the plasma. The arcing can lead to production of excessive deposition material in the form of splats or blobs. Reducing the content of conductivity anomalies and strengthening the to-be-deposited material is seen to reduce production of such splats or blobs. Other splat limiting steps include smooth finishing of the target surface and low-stress ramp up of the plasma.

This application is a continuation of Ser. No. 08/979,192, filed Nov.26, 1997, now U.S. Pat. No. 6,001,207. The disclosure of saidapplication is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The invention relates generally to physical vapor deposition (PVD) ofmetal films. The invention relates more specifically to DC magnetronsputtering of metals such as aluminum (Al) or aluminum alloys ontosemiconductor substrates and the like for forming fine pitchmetallization such as the electrically-conductive interconnect layers ofmodern integrated circuits.

2. Cross Reference to Related Patents

The following U.S. patent(s) is/are assigned to the assignee of thepresent application, and its/their disclosures is/are incorporatedherein by reference:

(A) U.S. Pat. No. 5,242,566 issued Sept. 7, 1993 to N. Parker; and

(B) U.S. Pat. No. 5,320,728 issued Jun. 14, 1994 to A. Tepman.

3. Description of the Related Art

The electrically-conductive interconnect layers of modern integratedcircuits (IC) are generally of very fine pitch (e.g., 10 microns orless) and high density (e.g., hundreds of lines per square millimeter).

A single, small defect in the precursor metal film that ultimately formsa metallic interconnect layer of an IC can be so positioned as toseriously damage the operational integrity of the IC. As such it isdesirable to form metal films with no defects or as few, minimally sizeddefects as possible.

The metal films of integrated circuits are typically formed by physicalvapor deposition (PVD). One low cost approach uses a DC magnetronapparatus such as the Endura™ system available from Applied MaterialsInc. of California for sputtering aluminum (Al) or aluminum alloys ontosemiconductor wafers.

Although such DC₋₋ magnetron PVD systems generally produce high qualitymetal films with relatively low defect densities, heretofore unexplained`blobs` of extra material are occasionally observed in the depositedmetal. These blobs can interfere with device formation anddisadvantageously reduce mass production yield of operable devices.

The present inventors have isolated such blobs in DC₋₋ magnetron-formedaluminum films, have analyzed the composition and physical structures ofsuch blobs, and have developed methods for minimizing the formation ofsuch undesirable blobs.

SUMMARY OF THE INVENTION

The above-mentioned problems are overcome in accordance with theinvention by providing an improved target for use in magnetronsputtering of aluminum, or of aluminum alloys or of like metals wherethe formed metal films having low defect densities.

It has been determined that the microscopic make up of the target in aDC₋₋ magnetron PVD system plays an integral role in the mechanisms thatlead to blob formation.

More particularly, nonhomogeneous structures within the target such asdielectric inclusions (e.g., Al₂ O₃ precipitates) and nonconductivevoids (e.g., formed by trapped gas bubbles), when exposed as part of thetarget surface, are believed to create corresponding distortions in theelectric fields that surround the target surface during the sputteringprocess. It is believed that large-enough distortions can evolve intopoints of field breakdown through which arcs of high current flowbetween the plasma and the target. Such arcing currents can result inlocalized melting of the target material and in the production ofrelatively large blobs of liquid material that splatter onto the wafersurface. The splattered material apparently draws back together oncontact with the wafer surface, due to surface tension effects, andsolidifies into the undesirable blob.

In accordance with a first aspect of the invention, targets aremanufactured so as to minimize the sizes and numbers of dielectricinclusions (e.g., Al₂ O₃ precipitates) and nonconductive voids (e.g.,formed by trapped hydrogen bubbles).

Blob formation is additionally believed to be due to stress-inducedbreakdown of the target material when the sputtering plasma is struck.The electric fields and currents which develop near the surface of thetarget as the plasma is ignited tend to generate mechanical stresseswithin the target material. Localized breakdown due to poor mechanicalstrength of the target local material is believed to be another sourceof blob generation.

In accordance with a second aspect of the invention, targets aremanufactured so as to homogeneously maximize the strength of the targetmaterial and thereby inhibit blob generation due to localized mechanicalbreakdown.

A target in accordance with the invention essentially excludesdielectric inclusions such as metal oxides (Al₂ O₃), nitrideprecipitates, carbide precipitates, of sizes larger than about 1 micronin concentrations greater than 5,000 such inclusions per gram of targetmaterial. A target in accordance with the invention alternatively orfurther essentially excludes voids such as those caused by entrapped gasof sizes larger than about 1 micron in concentrations greater than 5,000such voids per gram of target material. A target in accordance with theinvention alternatively or further has an essentially homogeneousdistribution of metal grain size in the range of about 75 micron and 90micron. A target in accordance with the invention alternatively orfurther has an initial surface roughness of less than about 20microinches.

A DC₋₋ magnetron PVD system in accordance with the invention comprises atarget having one or more of the following characteristics: (a)essentially no dielectric inclusions such as metal oxides (Al₂ O₃),nitride precipitates, carbide precipitates, of sizes larger than about 1micron in concentrations greater than 5,000 such inclusions per gram oftarget material; (b) essentially no voids such as those caused byentrapped gas of sizes larger than about 1 micron in concentrationsgreater than 5,000 such voids per gram of target material; (c) anessentially homogeneous distribution of metal grain size in the range ofabout 75 micron and 90 micron; and (d) an initial surface roughness ofless than about 20 microinches. A DC₋₋ magnetron PVD system inaccordance with the invention further comprises means for ramping plasmapower at a rate of 2Kw per second or less.

A target manufacturing method in accordance with the invention comprisesone or more of the following steps of: (a) obtaining purified aluminumhaving less than about 1 ppm of hydrogen and less than about 10 ppmoxygen; (b) casting the purified aluminum using a continuous-flowcasting method wherein the melt skin is not exposed to an oxidizingatmosphere; (c) working the cast metal so as to produce an essentiallyhomogeneous distribution of metal grains of diameters less than or equalto 100w and second phase precipitates of diameters in the range of about1 to 10 μ and more than about 50% material having <200> texture; (d)smoothing the initial target surface to an average roughness of no morethan about 20 microinches; (e) using ultrasonic cleaning to removearc-inducing contaminants from the initial target surface; and (f)shipping the cleaned target in an inert gas pack.

A method for operating a DC₋₋ magnetron PVD system in accordance withthe invention comprises the steps of: (a) installing a new target havingone or more of the following characteristics: (a) essentially nodielectric inclusions such as metal oxides (Al₂ O₃), nitrideprecipitates, carbide precipitates, of sizes larger than about 1 micronin concentrations greater than 5,000 such inclusions per gram of targetmaterial; (b) essentially no voids such as those caused by entrapped gasof sizes larger than about 1 micron in concentrations greater than 5,000such voids per gram of target material; (c) an essentially homogeneousdistribution of metal grain size in the range of about 75 micron and 90micron; and (d) an initial surface roughness of less than about 20microinches. A DC₋₋ magnetron PVD operating method in accordance withthe invention further comprises ramping plasma power at a rate of nomore than 2Kw per second or less.

Other aspects of the invention will become apparent from the belowdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The below detailed description makes reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic diagram of a DC sputtering magnetron;

FIG. 2 is a micrograph showing a cross sectional side view of anisolated `splat` or `blob` within the interconnect structure of anintegrated circuit;

FIG. 3 is a flow chart showing steps taken in the manufacture andsubsequent use of a target, including improvement steps in accordancewith the invention;

FIGS. 4A and 4B respectively show cross sectional views of a simplecasting process and the resultant product for purpose of explanation;and

FIG. 5 is a plot showing average splats number per wafer over sampledgroupings of 45 wafers each taken every 200 Kw hours of a sample targetin accordance with the invention.

DETAILED DESCRIPTION

FIG. 1 shows a schematic diagram of a DC sputtering magnetron system100. A magnet 110 is positioned over a portion of target 120. The targetincludes a deposition-producing portion that is electrically conductiveand is composed of the to-be-sputtered material (e.g., a metal such asaluminum). Target 120 is typically of a symmetrical form such as acircular disk, but may have various bends or other features such asshown for adaptively fitting into a specific DC₋₋ magnetron PVD systemand for producing specific distributions of electrical field intensityand gas flow in accordance with design specifics of the receiving DCmagnetron PVD system. The target 120 is typically structured forremovable insertion into the corresponding DC₋₋ magnetron PVD system100. Targets are periodically replaced with a new targets given that thePVD process erodes away the to-be-deposited material of each target.

A switching means 125 may be provided for selectively connecting thetarget 120 to a relatively negative voltage source 127. In general, thenegative voltage source 127 provides a DC cathode voltage in the rangeof about -470 V to -530 V relative to the potential on an opposed anode(ground or GND in the illustrated example). The specific cathode voltagevaries according to design. When switching means 125 is closed toconnect the target 120 with negative voltage source 127, the target canact as a source of negatively charged particles such as 135 (e⁻⁻) and138 (Al⁻⁻)which are discussed below. Because of this the target is alsoreferred to as the cathode.

A tubular gas-containment shield 130, usually of cylindrical shape, isprovided below and spaced apart from the target 120. Shield 130 iselectrically conductive and is generally coupled to ground (GND) or toanother relatively positive reference voltage so as to define anelectrical field between the target 120 and the shield. Shield 130 has aplurality of apertures 132 defined through it for admitting a suppliedflow of gas 131 such as argon (Ar) from the exterior of the shield 130into its interior.

A workpiece-supporting chuck 140 is further provided centrally below andspaced apart from the target 120, usually within the interior of theshield 130. Chuck 140 is electrically conductive and is generally alsocoupled to ground (GND) or to another relatively positive referencevoltage so as to define a further electrical field between the target120 and the chuck.

A replaceable workpiece 150 such as a semiconductor wafer is supportedon the chuck centrally below the target 120. Workpiece 150 originallyconsists of a substrate 152 having an exposed top surface 152a. As PVDsputtering proceeds, a metal film 155 having a top surface 155a buildsup on the substrate 152. It is desirable that the build up or depositionof the metal film 155 be uniform across the entire top surface 152a ofthe substrate, but as explained herein, anomalies sometimes interferewith homogeneous deposition.

Workpiece substrate 152 may include an insulative layer composed forexample of SiO₂. In such cases, the metal film 155 may be electricallyinsulated from chuck 140 and the voltage of the metal film 155 willfloat to a slightly negative level relative to the chuck's voltage(e.g., GND).

DC₋₋ magnetron operation initiates as follows. When switching means 125is closed, initial electric fields are produced between the target 120and the shield 130 and the chuck 140. Plasma igniting gas is introduced.The illustrated assembly of FIG. 1 is usually housed in a low pressurechamber 105 (partially shown) that maintains an internal pressure in therange of about 2 to 5 Torr or lower. Some of the supplied gas 131 thatenters the interior of shield 130 disassociates into positively chargedions (Ar⁺) and negatively charged ions (Ar⁻) when subjected to theinitial electric fields. One so-generated positive ion is shown at 133.Due to electrostatic attraction, ion 133 (Ar⁺) accelerates towards andcollides with the bottom surface of the target at first collision point,say 134. The point of collision is denoted with an asterisk ("*"). Thisinitial collision induces emission of an electron (e⁻⁻) 135 from cathode120. (A particle of target material (Al) may also be dislodged bycollision 134.) The emitted electron 135 drifts down towards the morepositive chuck 140. However, the magnetic fields of magnet 110 giveelectron 135 a spiraling trajectory as indicated at 136. Eventuallyelectron 135 collides with a molecule of the inflowing gas 131 (e.g.,Ar₂). This second collision (*) produces another positively charged ion137 (Ar⁺) which accelerates towards and collides with the bottom surfaceof the target. This third collision produces yet another electron like135, and a chain reaction is established leading to the creation of asustained plasma 160 within the interior of the gas-containment shield130. Plasma 160 is charged positive relative to the cathode 120 andbegins to act like a floating anode. This changes the electric fielddistribution within the DC₋₋ magnetron PVD system 100. At some point theelectric field distribution stabilizes into a long term steady state.

The ballistic collisions of massive particles such as 137 (Ar⁺) with thebottom surface of the target 120 sometimes cause small particles of thetarget's material to break off and move toward the underlying workpiece150. An example of such an emitted target particle is shown at 138. Thesizes and directions of the emitted target particles tend to produce arelatively uniform deposition of the emitted material (e.g., aluminum)on the top surface (152a and later 155a) of the workpiece 150.

On occasion, however, as explained above, the deposition is not uniformin that blobs or `splats` appear on or within the deposited metal film155. Some of the splats can have diameters as large as 500 microns,which is quite large in a world where operational features of theaffected device have dimensions of 1 micron or less. Such splats arcundesirable.

FIG. 2 is a micrograph taken with a focused ion beam (FIB) microscope ata magnification sufficient to show an anomalous section of a 1micron-thick aluminum line. The micrograph shows a cross sectional sideview of an isolated `splat` or `blob` within the interconnect structureof an integrated circuit. The splat diameter is approximately 5 micronsand the splat height is roughly 1.5 microns. In this particular case,the deposited metal film (155) is an alloy of aluminum and copper(AlCu). The `splat` is given its name because of the appearance thatsomething had splattered onto the otherwise planar, PVD deposited metalfilm.

In the captured splat of FIG. 2, an inclusion having a diameter ofroughly 0.3 micron is seen within the body of the splat. Inclusions arenot routinely observed in every splat. Inclusions such as the one shownin FIG. 2 were isolated and analyzed chemically. The analysis showedthat such inclusions were composed primarily of the oxide, Al₂ O₃.

The present inventors deduced that the Al₂ O₃ inclusion had come fromthe specific target (120) used in the PVD sputtering process, given thatthe feed gas consisted of relatively pure Ar, and the substrate had beencleaned, and there was nothing else in the DC₋₋ magnetron PVD systemthat could act as a source of Al₂ O₃.

The present inventors further deduced that the Al₂ O₃ inclusion was acausal factor in the generation of the observed splat even though thesplat is further defined by an excessive amount of AlCu that appears tohave splattered onto the forming metal film during the PVD process.

Further questions remained, however. What was the specific mechanismthat made the Al₂ O₃ inclusion a causal factor in the generation of theobserved splat? Why do many splats not include such Al₂ O₃ inclusions?

It is believed that the correct answer lies in understanding howelectrical fields are distributed within the DC₋₋ magnetron system 100(FIG. 1) after plasma 160 reaches steady state stability, and how thisstability can be temporarily disturbed.

Referring to FIG. 1, as the plasma 160 reaches steady state stability,there develops between the bottom surface 120a of the target and a topboundary 160a of the plasma, an area that is essentially free ofelectrons or other charged particles. This charge-free region isreferred to in the literature as the `dark space` or the `dead space`.Its extent is referenced in FIG. 1 as 170 (not to scale).

A relatively large voltage differential develops between the top 120aand bottom 160a of the dark space 170. Target-emitted electrons such as135 are believed to tunnel rather than to drift through the dark space170 and to thereby maintain the relatively large voltage differentialbetween the top and bottom of the dark space 170.

A relatively homogenous distribution of electric field intensity isgenerally needed along planes 160a and 120a to maintain the continuityof the dark space 170. (Lower plane 160a is referred to as a virtualanode surface.)

It is believed that pinhole-like breaches in the continuity of the darkspace 170 occur from time to time. A breach may occur because of alocalized increase in electric field intensity. The latter causal effectmay come about because a discontinuity develops in the localizedconductivity of one or both of the cathode surface 120a or virtual anodesurface 160a. If the size of the breach is significant, a sudden rush ofcharged particles may pass through the breaching pinhole, from theplasma 160 into the target 120. In essence, an arc of current ofrelatively large magnitude, can pass between the plasma 160 and thetarget 120 at the point of breach of the dark space 170.

If a sufficiently large arc is produced, a significant amount of heatmay be generated at or around the arc-struck point of the target'ssurface 120a. Localized temperature may rise sufficiently to melt anarea about the arc-struck point. The molten target material can separatefrom the target and become drawn to the more positively charged chuck140. When the molten target material hits the top surface 155a of theworkpiece, it splatters, cools, and adheres to the top surface 155a asan anomaly.

Computer simulations have shown that dropping a glob of molten metalonto a planar, solid metal surface produces a dome-shaped blob ofmaterial having ripples of the type seen in the `splat` of FIG. 2 on theplanar surface. Re-consolidation of the splattered material occurs dueto surface tension and cooling of the splattered blob. The blob ofanomalous material re-consolidates and solidifies into the rippled,dome-shaped form. This supports the present inventors' hypothesis thatsome splats are produced by a melting of material on the target'ssurface 120a.

The present inventors suspect that localized melting is not the onlymechanism by which nonhomogeneous deposition of excessive targetmaterial occurs onto the workpiece surface 155a. An arc-struck part ofthe target's surface 120a might be mechanically weak. The shock orresultant thermal stress of a current arc may dislodge the mechanicallyweak part from the target's surface. The dislodged but not necessarilymolten material can then be drawn to the workpiece top surface 155a toform a nonhomogeneous, excessive deposition at the point of landing.

Al₂ O₃ inclusions are electrically nonconductive or of high electricalresistance, and as such they define discontinuities in the voltagedistributing or conductive properties of the target's bottom surface120a. Internal Al₂ O₃ inclusions within the target become part of thetarget's surface as the surface 120a is eroded away by ion bombardmentto expose the previously internal inclusions.

Abrupt changes in the localized intensity of electric fields neighboringthe bottom surface 120a of the target can develop when sufficientlylarge Al₂ O₃ inclusions, or other forms of disruption in the electricalconductivity properties of the target's bottom surface 120a becomeexposed. This can lead to breach of the dark space 170, arcing, and theproduction of molten blobs or mechanically-dislodged anomalies. Ingeneral, such regions of disruption in the electrical conductivityproperties of the target's bottom surface 120a are referred to herein asconductivity anomalies. A conductivity anomaly of relatively highelectrical resistance is defined as a region having a resistivity atleast 100 times greater than a corresponding electrical resistivity ofan anomaly-free representative portion of the to-be-deposited metal.

In view of the above, and in accordance with one aspect of theinvention, it is desirable to minimize the numbers and sizes ofconductivity anomalies within the to-be-deposited material of the target120. Aside from oxides such as Al₂ O₃, conductivity anomalies caninclude nitride precipitates, carbide precipitates, contaminants thatproduce cathodic vapor bursts, and voids in the metal, where the lattervoids may be originally defined by trapped gas bubbles.

It is to be understood that when the composition or othercharacteristics of the `target` is discussed herein, that discussion isprimarily directed to portions of the target that are bombarded byplasma-produced ions and are possibly subjected to being struck by arcsand as a result producing anomalous depositions. Targets in general mayhave additional portions that are adapted for replaceable receipt intoand/or electrical coupling with the remainder of the DC₋₋ magnetron PVDsystem. Those additional portions may not require special compositioningor structuring in accordance with the present invention in instanceswhere those additional portions are not bombarded by plasma-producedions.

In accordance with another aspect of the invention, it is desirable tomaximize the microhardness (and thereby the micro-strength) of thetarget material so that arc-struck parts of the target are preventedfrom being so mechanically weak as to allow arc-induced dislodging ofsuch target parts.

Disruptions in the uniformity of electric field intensity about thebottom surface 120a of the target can also come about due to excessiveroughness in the initial form of that bottom surface 120a.

In accordance with yet another aspect of the invention, it is desirableto minimize the roughness of the initial bottom surface 120a of thetarget so as to inhibit disruptions in the uniformity of electric fieldintensity about the dark space 170.

Disruptions in the uniformity of electric field intensity about thebottom surface 120a of the target can also come about due to excessivedirt being left on the initial form of that bottom surface 120a when thetarget is first used (burnt in). The dirt can induce arcing. The lattercan produce pits or other unevenness in the target surface which thenproduces yet more arcing.

In accordance with yet a further aspect of the invention, it isdesirable to minimize dirt on the initial bottom surface 120a of thetarget so as to inhibit dirt-induced arcing.

An aluminum target in accordance with the invention has one orpreferably more of the following homogenous characteristics of Table 1:

                  TABLE 1                                                         ______________________________________                                        PROPERTY           PREFERRED RANGE                                            ______________________________________                                        Dielectric Inclusion                                                                             less than about                                              Content, where such 5000 inclusions per gram                                  inclusions have widths of of target material                                  0.3 micron or more                                                            Hydrogen content less than about 0.5 ppm                                      Carbon content less than about 10 ppm                                         Oxygen content less than about 10 ppm                                         Nitrogen content less than about 10 ppm                                       Metal grain size less than about 100                                           micron                                                                       (200) textured material greater than 50%                                      (111) textured material less than about 3%                                    Hardness greater than about                                                    50 (Rockwell scale)                                                          Surface roughness less than about 20 micro-                                    inches                                                                       Alloy strengthening greater than about 0.5%                                   addend Cu by weight                                                           Alloy precipitate size about 5 microns or less                                Other impurities less than about 10 ppm                                     ______________________________________                                    

Looser requirements can also be adapted for Table 1. For example: thenumber of allowed inclusions per gram of target material can be widenedto 7,500 or 10,000; the definition of to-be-limited inclusions can bebroadened to those having widths of about 1 micron or more; and theallowed hydrogen content can be loosened to less than about 1 ppm (partsper million).

Another, more stringently-controlled, aluminum target in accordance withthe invention has one or preferably more of the followingcharacteristics of Table 2:

                  TABLE 2                                                         ______________________________________                                        PROPERTY          PREFERRED RANGE                                             ______________________________________                                        Dielectric Inclusion                                                                            less than about                                               Content, where such 5000 inclusions per gram                                  inclusions have widths of of target material                                  0.1 micron or more                                                            Hydrogen content less than about 0.075 ppm                                    Carbon content less than about 5 ppm                                          Oxygen content less than about 10 ppm                                         Nitrogen content less than about 7 ppm                                        Metal grain size between about 75 micron                                       and 90 micron                                                                (200) textured material greater than 75%                                      (111) textured material less than about 1%                                    Hardness greater than about                                                    50 (Rockwell scale)                                                          Surface roughness less than about 16 micro-                                    inches                                                                       Alloy strengthening about 0.5% Cu by weight                                   addend                                                                        Alloy precipitate size less than about 4 microns                              Other impurities less than about 5 ppm                                      ______________________________________                                    

Even tighter requirements can also be adapted for Table 2. For example:the number of allowed inclusions per gram of target material can benarrowed to 3,000 or 1,000; the definition of to-be-limited inclusionscan be tightened to include those having have widths of about 0.5 micronor more; the allowed hydrogen content can be tightened to less thanabout 0.05 ppm (parts per million), the allowed initial surfaceroughness can be reduced to 10 microinches or less; and the requiredamount of <200> texture material can be raised to 90% or more.

Referring to FIG. 3, the manufacturing steps by which targets inaccordance with the invention can be realized are discussed.

FIG. 3 is a flow chart showing steps taken in the manufacture andsubsequent use of a target in accordance with the invention. The overallmanufacture-and-use process is referenced as 300.

At step 301, the raw materials that will form the target are acquiredthrough mining or other means. It is desirable to acquire the rawmaterials from appropriate sources so that the acquired raw materialshave minimal amounts of initial impurities, particularly oxygen (O),hydrogen (H), nitrogen (N), carbon (C), and silicon (Si) in the recitedorder.

Minimizing initial O content is especially desirable because such oxygencontent can lead to later formation of undesirable metal oxides such asAl₂ O₃ inclusions. Minimizing N and C content is less but stilldesirable because the inclusion of significant amounts of these elementscan lead to later formation of undesirable insulative inclusionscomposed of metal nitrides and/or metal carbides. Silicon can combinewith any of O, N and C to form dielectric inclusions, and as such itscontent should also be minimized. Hydrogen can form hydrogen bubblesthat become trapped within the alloy melt during casting (see step 303described below).

Improvement step 321 (minimizing initial impurities) can be carried outby selecting a mine or other source of raw materials that meets thecriteria discussed above or by using a supplier further on the verticalproduction chain that uses products of raw materials obtained in thisway.

At step 302, the acquired raw materials are purified to produce aninitial form of the target metal. The target metal can be aluminum or analloy thereof such as Al_(x) Cu_(y) Si_(z) where x+y+z=100% and x>>y+z.

Some purification methods tend to generate more insulative or highresistance anomalies than others. Improvement step 322 is to select apurification method that minimizes generation of insulative or highresistance anomalies. The Hall-Bayert aluminum purification method isone example of such an inclusions minimizing process. The supplier ofthe purified target metal should be selected according to whether theyuse, or instructed to use, a purification method that minimizesgeneration of insulative or high resistance inclusions.

Examples of purified metals that conform with the above include, but arenot limited to high quality, aluminum available from Pechineg Inc.; andfrom Sumimoto Inc. of Japan.

At step 303, the purified materials are melted and cast, typically in acasting crucible.

Referring to FIG. 4A, the casting process can introduce large amounts ofinsulative or high resistance inclusions or voids if an appropriatecasting method is not selected. FIG. 4A shows a cross sectional view ofa simple, no-flow casting process 400 that, in accordance with theinvention, should be avoided. Undesirable casting method 400 is shownfor purpose of explaining why other processes are more desirable inaccordance with the invention.

In FIG. 4A, a crucible 410 is provided coupled to a heat source 420.Crucible 410 is typically made of a ceramic such as graphite. Solidinput material (e.g., purified aluminum) is placed inside the crucible.Heat is applied from the heat source 420 to produce a liquid melt 430 ofthe input material. The top surface of the melt 430 is exposed to anatmosphere 440 containing one or more of hydrogen (H₂), oxygen (O₂)water vapor (H₂ O) and other gases (e.g., N₂, CO₂) or other vapors.

Because of the high temperatures involved, the top surface of the melt430 reacts with the atmosphere 440 to produce an oxide skin 445 at theinterface between the melt 430 and the atmosphere 440. The oxide skin445 is relatively thin compared to the volume of the melt 430. As such,one would not expect to find great incorporation of the oxide materialinto the melt 430. However, convective flow currents 435 may beestablished within the melt. These currents 435 may work to suck inalready-produced oxides from the skin 445 toward the center of the meltand to expose fresh melt material to the oxidizing atmosphere 440. Assuch significant quantities of oxides (significant for our purposes) canbe incorporated into the melt 430 during this simple, one-shot, no-flow,casting method 400.

Also, due to wear and tear over time, additional material 415 such asgraphite particles can flake off from the interior walls of the cruciblefor incorporation within the melt 430. These flakes 415 can producecarbides or other insulative inclusions within the melt.

Also, because liquified metals such liquid aluminum tend to absorbhydrogen (H₂) rather easily when hot, such gases can be absorbed fromthe atmosphere 440 and dissolved into the melt 430 in significantquantities. The same gases (e.g., hydrogen) may not be as soluble in themetal as the metal later cools. When large vats of molten metal arecooled, the earlier absorbed gases (e.g., hydrogen) can become trappedin the form of gas precipitates or bubbles. These are later seen asvoids in the solid mass of the cast metal.

Referring to FIG. 4B, it is seen that after pouring and cooling, thecast metal 450 (e.g., cast Al) can contain large amounts of oxideinclusions 450a, trapped hydrogen bubbles 450b and other anomalies 450c(e.g., crucible flakes) that were introduced during the illustratedcasting process 400. If and when such anomalies 450a, 450b, 450c becomepart of the bottom surface 120a of the target (FIG. 1), they can inducearcing between the plasma 160 and the target 120, and the latter canresult in the generation of undesirable blobs or splats within the PVDdeposited metal 155.

In accordance with the invention, alternative casting methods should beused to minimize convective incorporation of gases, oxides and crucibleflakes into the melt. In general these alternate methods rely oncontinuous flow casting rather than on noncontinuous casting wherein amelt is allowed to develop convective currents. In general thesealternate methods further rely on in-vacuum or plunger-covered processesrather than those that expose the melt to an atmosphere 440 containinginclusion-producing moieties such as oxygen and hydrogen.

Included in these alternate, casting methods are the continuous flowmethods used by Johnson Matther Corp. of Spokane, Washington, thecontinuous flow methods used by Tosoh Corp. of Columbus, Ohio, and thein-vacuum flow casting methods used by Japan Energy Corp. of Tsukuba,Japan. The cast metal produced by these alternate methods tend toexhibits low amounts of inclusion content in accordance with theinvention.

Improvement step 323 of FIG. 3 indicates that one or more of thefollowing methods should be used in the casting step 303: (1) minimizeabsorption of H₂ and/or other gases from atmosphere by using in-vacuumor plunger-covered methods; (2) reduce crucible introduction of flakesinto the melt-by periodically scrapping loose material off the castingcontainer walls or using other techniques for preventing flaking; (3)use continuous-flow rather than in-place casting methods so as toprevent convective incorporation into the melt of oxidized skin material445.

Continuing with FIG. 3, after the casting process, the metal istypically worked by forging, rolling, deforming, or other metal workingtechniques as indicated at step 304. The metal working step 304 canalter any one or more of the texture, hardness, alloy precipitatedistributions, and other such attributes of the worked-on metal.

In accordance with improvement step 324 of the invention, the post-workmetal texture should be at least 50% of the <200> texture and less than396 of the [111] texture. As is known in the art, <200> texture enhancesuniformity of PVD deposition.

In further accordance with improvement step 324 of the invention, thepost-work metal texture should also have a homogeneously-distributedmicrohardness of substantially more than 20 Rockwell, preferably ahomogeneously-distributed microhardness of at least 40 Rockwell.Hardness correlates with strength. The homogeneously-distributedmicrohardness of a target in accordance with the invention providesgreater localized strength and thus greater immunity to melting or otherdislodging of target material due to arcing.

In further accordance with improvement step 324 of the invention, thepost-work metal texture should also have a homogeneously-distributedcontent of second-phase hardening alloy precipitates such as of theAl_(x) Cu_(y) Si_(z) formulation, where x+y+z=100% and x>>y+z, andfurther where the second-phase hardening alloy precipitates primarilyhave widths in the range of 1 micron to 10 microns. Al₄ Cu₉ is anexample of such second-phase hardening alloy precipitates.

Of course, for such work-induced generation of second-phase hardeningalloy precipitates, the melt material should already include theappropriate alloy constituents such as copper or silicon. For PVDdeposition of interconnect metal used in integrated circuits, theselection of alloy constituents should also conform to the limitationsof the metal etch processes used after PVD. In general, for currentlyused metal etching processes, the copper content should be equal to lessthan about 0.5% by weight and the silicon equal to less than about 1% byweight.

In further accordance with improvement step 324 of the invention, thepost-work metal should also have a homogeneously-distributed content ofsmall metal grains primarily having widths less than about 100 microns.

Machining step 305 is carried out conventionally to define the shape ofthe target.

For the following surface treatment step 306, the improvement is tosmooth the target's initial surface to a roughness that is no more than,and preferably less than 20 micro inches. Polishing to a greatersmoothness helps to reduce the number of sharp surface features thatmight act as field emission points and helps to keep the electric fieldintensity smooth. Arcing may begin at the tips of field emission pointsthat emit electrons.

Surface polishing is followed by cleaning and shipping step 307. Theimprovement step 327 includes the use of ultrasonic cleaning forremoving dielectric contaminants from the polished surface. It isbelieved that dielectric surface contaminants can define initial arcingspots. Initial arcing can occur when the target is first installed intothe sputtering chamber and burned in. Such initial arcing can result inlong-term damage to the target's otherwise smooth surface. The resultantroughness can produce further arcing and undesired splats after burn-incompletes.

After the ultrasonic cleaning, the target should be packaged in ahydrophobic, non-outgassing package containing an inert gas such asargon (Ar). These clean conditions should be continued as the target isinstalled into the sputtering chamber. The reason for such clean-roomstyle handling of the target is to again prevent contamination of thetarget's initial surface with arc-inducing substances.

Step 308 refers both to the initial burn-in of the target and itssubsequent long-term use. During burn-in, the plasma is used to removeany remaining contaminants from the target surface while dummy waferspass through the sputter chamber. Some recipes call for rapid burn-inwherein the full operational power level (e.g., 10 Kw) is immediatelyapplied to the target. It is believed that such rapid burn-in candisadvantageously stress the target mechanically and produce weak spotsand consequential splat generation. In accordance with improvement 328,electromechanical stresses to the target which result from rapid plasmaignition are avoided as much as possible. Target burn-in should be takenin slow increments such as first applying only one Kw or less to thetarget for an initial burn-in, then applying 3 Kw for a second burn-inperiod, then 5 KW and finally, the production level power which in thisexample is 10.6 Kw. During burn-in, sample wafers are run through thesputtering chamber and the reflectants of the sputtered-on aluminumfilms is measured. The optical reflectants of the aluminum film improvesas the burn-in process removes surface contaminants from the target.Burn-in is deemed to be complete when the reflectants value stabilizesat about 200% of the reflectants value of the corresponding silicon.

The same improvement 328 is further applied during production sputteringeach time the plasma is ignited. Rather than having rapid ignition ofthe plasma from no power to the full production level power (e.g., 10.6Kw), it is preferable to slowly ramp up the ignition power at, say, 2 Kwper second, and more preferably at 0.5 Kw/sec to 1.0 Kw/sec or lesswhile chamber pressure is modulated. Power ramp down can be at a moreaccelerated rate of say 3 Kw per second or less and chamber pressureshould be kept low during this time to exhaust loose particles. The slowpower ramp ups and ramp downs tend to reduce the amount ofelectromechanical stress applied to the target over unit time. This isbelieved to reduce the number of weak spots produced in the target as aresult of mechanical stressing. Additionally, slow ramp downs tend toreduce end-of-process re-adhesion of material to the target. Re-adheredmaterial can increase surface roughness or create splat-generatingpoints in other ways. In accordance with the invention therefore, plasmapowering means 125 (FIG. 1) includes means for ramping power up to theplasma at a rate of 2 Kw per second or less and ramping down power tothe plasma at a rate of 3 Kw per second or less.

FIG. 5 is a plot showing splat generation density over the life of atarget produced in accordance with the invention. This particular targethad the following characteristics: there were essentially no dielectricinclusions such as metal oxides (Al₂ O₃), nitride precipitates, carbideprecipitates, of sizes larger than about 1 micron in concentrationsgreater than 5,000 such inclusions per gram of target material; therewere essentially no voids such as those caused by entrapped gas of sizeslarger than about 1 micron in concentrations greater than 5,000 suchvoids per gram of target material; an essentially homogeneousdistribution of metal grain size in the range of about 75 micron and 90micron was provided; an initial surface roughness of less than about 20microinches was provided.

The average number of splats per wafer (where a splat is defined as anabnormality greater than 1 micron in diameter) was measured over lots of45 wafers each with samples being taken every 200 Kw hours of usage ofthe target. Each wafer was 200 mm in diameter. The peak power levelduring sputtering was 10.6 Kw produced at 2 mTorr pressure with atarget-to-wafer spacing of 52 mm. The goal was to produce less than fivesplats per wafer on average. As seen in FIG. 5, the average splatdensity remained at less than two splats per wafer for the first 600KwHrs of target life and climbed to approximately three splats per waferaverage after approximately 800 KwHrs of target life.

In contrast to FIG. 5, conventional production targets which were notformulated in accordance with the tight process controls of the presentinvention tended to produce 10 or more splats per wafer on average. Thetest results of FIG. 5 show that tight control of target manufacturesignificantly reduces the average number of splats per wafer.

Testing of targets for conformance with the above criteria may becarried out as follows. Inclusion content of the target may be measuredusing a wet chemical dissolution technique. In one such methodpolyethylene beakers are thoroughly cleaned before use. Acids andreagent water are filtered through 0.45 micron diameter, membranefilters before use. Sample aluminum targets are rough cut by saw tosample sizes such as 1 gram each, then finished to 240 grit on polishingwheels. The samples are then precleaned by dipping in a separate bath of30% HCl for a short time (e.g., 5 seconds) just before full dissolution,in order to remove any traces from the grinding. The samples arethereafter dissolved to their full extent in a clean aqueous solutionhaving 30% HCl at room temperature or higher. 100 mL is used in the caseof 1 gram samples, and 500 mL is used for 10-30 gram samples. Solids arecollected out of the HCl solution on 0.45 micron gridded diameterfilters for optical microscopy/SEM analysis, and on 0.22 micronungridded 47 mm diameter filters for chemical analysis. Copper isdissolved off using a 10% HNO₃ wash on the filters. All these operationsshould be carried out in a HEPA filtered laminar flow hood. The washedfilters are then allowed to dry in a class 100 clean room, beforemicroscopic examination. The inclusion size distribution may bedetermined using manual light microscopy techniques such as, ASTM F24and F25. Oblique lighting should be used to prevent contamination duringthe analysis.

Gas/void content and size distribution in aluminum target samples may bemeasured using LECO analysis(Liquid Emission Collemetry). Metalhardness, grain size, and other working-induced characteristics may bedetermined using conventional metal characterization techniques. Targetsfrom appropriately sampled lots that meet the criteria set forth abovemay then be designated as conforming in accordance with acceptedstatistical techniques. Targets from appropriately sampled lots that donot meet one or more of the criteria set forth above should be excludedfrom PVD metal operations where splat formation is a concern.

The above disclosure is to be taken as illustrative of the invention,not as limiting its scope or spirit. Numerous modifications andvariations will become apparent to those skilled in the art afterstudying the above disclosure.

By way of example, the production of conforming targets and conforminguse thereof may include purchase from a third party source of targetsthat have already passed through any one or more of say, steps 301-305(plus corresponding improvements 321-324) followed by the carrying outof the remaining steps, such as say 306-308 (plus correspondingimprovements 326-328). Sampling may be used to check statisticalconformance by third party suppliers to the specifications called for bythe respective improvement steps.

Given the above disclosure of general concepts and specific embodiments,the scope of protection sought is to be defined by the claims appendedhereto.

What is claimed is:
 1. A method for operating a DC magnetron PVD systemadapted to receive a target for installation therein, wherein saidtarget has a deposition-producing portion composed primarily of anelectrically conductive, to-be-deposited metal, said operating methodcomprising the steps of:(a) installing a target obtained in accordancewith a certification program wherein said certification programqualifies the target as having a content of less than about 10,000conductivity anomalies per gram of the to-be-deposited metal where eachsaid conductivity anomaly in said content is a region having adetectable size of at least 1 micron and where each said conductivityanomaly defines an insulative or high resistivity region, highresistivity being at least 100 times greater than a correspondingelectrical resistivity of an anomaly-free representative portion of theto-be-deposited metal; and (b) ramping up plasma power at an averagerate of no more than 2Kw per second.
 2. A method for operating a DCmagnetron PVD system according to claim 1 further comprising the stepof:(c) ramping down plasma power at an average rate of no more than 3Kwper second.
 3. The DC Magnetron PVD operating method of claim 1 whereinsaid certification program qualifies the target as having at leastfollowing characteristics (a.1) and (a.2):(a.1) less than about 10,000dielectric inclusions per gram of target material wherein saiddielectric inclusions have widths of 0.3 micron or more; and (a.2) ahydrogen content of less than about 0.5 ppm.
 4. The DC Magnetron PVDoperating method of claim 3 wherein said certification program furtherqualifies the target as having at least a following (a.3)characteristic:(a.3) an essentially homogeneous distribution of metalgrains each of a diameter less than about 100 microns.
 5. The DCMagnetron PVD operating method of claim 4 wherein said certificationprogram further qualifies the target as having at least a following(a.4) characteristic:(a.4) at least 50% material having {200} texturing.6. The DC Magnetron PVD operating method of claim 5 wherein saidcertification program further qualifies the target as having at least afollowing (a.5) characteristic:(a.5) an initial surface roughness on thedeposition-producing portion of less than 20 microinches.
 7. The DCMagnetron PVD operating method of claim 3 wherein said certificationprogram further qualifies the target as having at least a following(a.3) characteristic:(a.3) a carbon content of less than about 10 ppm.8. The DC Magnetron PVD operating method of claim 7 wherein saidcertification program further qualifies the target as having at leasttwo of following characteristics (a.4) through (a.9):(a.4) an oxygencontent of less than about 10 ppm; (a.5) a nitrogen content of less thanabout 10 ppm; (a.6) an essentially homogeneous distribution of metalgrains each of a diameter less than about 100 microns; (a.7) anessentially homogeneous distribution of second phase precipitates ofdiameters in the range of about 1 micron to 10 microns; (a.8) at least50% material having {200} texturing; (a.9) an initial surface roughnesson the deposition-producing portion of less than 20 microinches.
 9. TheDC Magnetron PVD operating method of claim 3 wherein said certificationprogram further qualifies the target as having:(a.1a) less than about5000 dielectric inclusions per gram of target material wherein saiddielectric inclusions have widths of 0.3 micron or more.
 10. The DCMagnetron PVD operating method of claim 3 wherein said certificationprogram further qualifies the target as having:(a.1a) less than about5000 dielectric inclusions per gram of target material wherein saiddielectric inclusions have widths of 0.1 micron or more.
 11. A DCmagnetron PVD system having a splat-limited target installed therein,wherein said target has a deposition-producing portion composedprimarily of an electrically conductive, to-be-deposited metal, and saidtarget satisfies at least following characteristics (a.1) and(a.2):(a.1) less than about 5000 dielectric inclusions per gram oftarget material wherein said dielectric inclusions have widths of 0.3micron or more; and (a.2) a hydrogen content of less than about 0.5 ppm;(b) power control means for ramping up plasma power at an average rateof no more than 2Kw per second.
 12. A DC₋₋ magnetron PVD systemaccording to claim 11 wherein said splat-limited target includesaluminum as a major component thereof.
 13. The DC magnetron PVD systemof claim 11 wherein said splat-limited target further satisfies at leasta following (a.3) characteristic:(a.3) an oxygen content of less thanabout 10 ppm.
 14. The DC Magnetron PVD system of claim 13 wherein saidsplat-limited target further satisfies at least a following (a.4)characteristic:(a.4) a nitrogen content of less than about 10 ppm. 15.The DC Magnetron PVD system of claim 14 wherein said splat-limitedtarget further satisfies at least a following (a.5) characteristic:(a.5)at least 50% material having {200} texturing.
 16. The DC Magnetron PVDsystem of claim 15 wherein said splat-limited target further satisfiesat least a following (a.6) characteristic:(a.6) an essentiallyhomogeneous distribution of metal grains each of a diameter less thanabout 100 microns.
 17. The DC Magnetron PVD system of claim 16 whereinsaid splat-limited target further satisfies at least a following (a.7)characteristic:(a.7) an essentially homogeneous distribution of secondphase precipitates of diameters in the range of about 1 micron to 10microns.
 18. The DC Magnetron PVD system of claim 11 wherein saidsplat-limited target further satisfies at least a following (a.3)characteristic:(a.3) a carbon content of less than about 10 ppm.
 19. TheDC Magnetron PVD system of claim 11 wherein said splat-limited targetfurther satisfies at least a following (a.3) characteristic:(a.3) atleast 50% material having {200} texturing.
 20. A method for using a DCMagnetron PVD system adapted to receive a target for installationtherein, wherein said target has a deposition-producing portion composedprimarily of an electrically conductive, to-be-deposited metal, saidmethod comprising the steps of:(a) installing a target in said DCMagnetron PVD system wherein said installed target is pre-qualified ashaving a content of less than about 5,000 conductivity anomalies pergram of the to-be-deposited metal where each said conductivity anomalyin said content is a region having a detectable size of at least 1micron and where each said conductivity anomaly defines an insulative orhigh resistivity region, high resistivity being at least 100 timesgreater than a corresponding electrical resistivity of an anomaly-freerepresentative portion of the to-be-deposited metal; and (b) applying aninitial burn-in plasma to the installed target to thereby removecontaminants from an exposed surface of said deposition-producingportion of the target, wherein power of said initial burn-in plasma isincremented over plural periods of time rather than being raisedimmediately to production level power.
 21. The system using method ofclaim 20 and further comprising the step of:(c) after said applicationof the initial burn-in plasma, repeatedly ramping down said plasma to anoff state and thereafter ramping the plasma back up to production levelpower at a ramp up rate of 2 Kw per second or less.
 22. The system usingmethod of claim 21 wherein:(c. 1) said ramping down of the plasma to theoff state is at a rate of 3 Kw per second or less.
 23. The system usingmethod of claim 20 wherein(a. 1) each said conductivity anomaly in saidcontent is a region having a detectable size of at least 0.3 micron andwhere each said conductivity anomaly is defined by a high resistivityinclusion or an insulative void.
 24. The system using method of claim 20wherein(a.1) respective ones of said conductivity anomalies are definedby at least one of a metal oxide, a nitride precipitate, a carbideprecipitate, a surface contaminant that produces cathodic vapor bursts,and voids in the deposition-producing portion of the installed target.25. A method for using a DC Magnetron PVD system adapted to receive atarget for installation therein, wherein said target has adeposition-producing portion composed primarily of an electricallyconductive, to-be-deposited metal that is to be deposited by said systemon supplied substrates, said method comprising the steps of:(a)installing a prequalified target in said DC Magnetron PVD system whereinsaid target is pre-qualified as having:(a.1) a content of less thanabout 10,000 conductivity anomalies per gram of the to-be-depositedmetal where each said conductivity anomaly in said content is a regionhaving a detectable size of at least 1 micron and where each saidconductivity anomaly defines an insulative or high resistivity region,high resistivity being at least 100 times greater than a correspondingelectrical resistivity of an anomaly-free representative portion of theto-be-deposited metal; and (a.2) an initial target surface that issmoothed to an average roughness of no more than about 20 microinches;and (b) prior to using said prequalified target for depositing the metalon non-dummy ones of the supplied substrates, cleaning the initialtarget surface to remove therefrom arc-inducing contaminants.